Disk drive devices are information storage devices that use magnetic media to store data and a movable read/write head positioned over the magnetic media to selectively read data from and write data to the magnetic media.
As shown in FIG. 1a, a conventional hard disk drive unit includes a magnetic disk 101 mounted on a spindle motor 103 for spinning the disk 101 at a constant high speed. A head gimbal assembly (HGA) 102 which includes a slider incorporating a read/write head (not shown) is actuated to move relative to the disk 101 so as to read data from or write data to the disk 101.
Typically, a VCM 150 is employed to actuate the HGA 102 so as to position the head with respect to the disk surface. The HGA 102 is mounted at a tip end of an actuator arm 104. The actuator arm 104 pivots about a cartridge bearing assembly 110 mounted to the disk drive base plate at a position closely adjacent to the outer extreme of the disk 101 so that the head moves in a plane parallel with the surface of the disk 101 and over data tracks of the disk 101.
The VCM 150 includes a coil 151 mounted radially outward from the cartridge bearing assembly 110, the coil 151 being immersed in the magnetic field of a magnetic circuit of the VCM 150. The magnetic circuit comprises one or more permanent magnet pairs and magnetically permeable plates. When a predetermined driving current flows through the coil 151, rotational forces or torques about the axis of the cartridge bearing assembly 110 are generated on the coil 151 as well as the actuator arm 104 by the interaction between the current and the magnetic field.
As shown in FIG. 1b, there are typically three principal torques experienced by the VCM and the actuator arm 104 as a result of the application of current to the coil 151. The first torque Mz (generated by force Fr as shown in the drawings), often called the main torque, causes the coil 151 and the actuator arm 104 to rotate about a z-axis 250 of the cartridge bearing assembly 110 in the plane X-Y. The second torque Mx (generated by force Ft as shown in the drawings), referred to as the torsion, causes the coil 151 and the actuator arm 104 to rotate or twist about an x-axis 254 of the cartridge bearing assembly 110. The third torque My, referred to as pitch torque, causes the coil 151 and the actuator arm 104 to rotate or bend about a y-axis 258 of the cartridge bearing assembly 110. As is known, the main torque Mz is the primary means by which the voice coil 151, and thus the head, is moved radially across the disk 101. Stated another way, the main torque Mz is a desired force which causes the actuator arm 104 and head to move in a plane parallel with the disk 101. In contrast, both the torsion Mx and pitch torque My cause motions in the actuator arm 104, the head, and the coil 151 which are not parallel to the plane of the disk 101. As such, the torques Mx and My affect the head slider's ability to maintain optimal flying height and to stay parallel to the disk over the data tracks, thereby interfering with the read/write operation of the head in the disk drive.
FIG. 2 shows a typical bode curve of a HSA. The curve can be viewed as the output/input ratio in frequency domain. The input is the electrical current applied on the coil, while the output is the lateral displacement between head and disk. The base line of the bode curve is a straight line with slope as −2, as shown by the dashed line. It can be observed that a peak exists at frequency 4 KHz, which corresponds to the coil torsion mode, and is caused mainly by Mx. This peak will adversely affect the slider's performance dramatically. Therefore, the coil torsion caused by the undesired torque Mx is very critical in VCM design, while the coil bend caused by the pitch torque My is less critical in VCM design.
Generally speaking, magnetic flux lines are representative of the magnetic fields generated by a permanent magnet or by a current flowing in a wire. With respect to permanent magnets, magnetic flux lines are typically represented by dashed lines that emerge from the magnet's north pole and enter the magnet's south pole. The density of the flux lines indicates the magnitude of the magnetic field generated by the magnet. If a magnetic conductive material, such as steel, is placed in a flux path, the magnetic flux will tend to pass through the steel rather than air surrounding the magnet, as the steel has a higher magnetic permeability.
Referring to FIG. 3b, FIG. 4b, FIG. 7 and FIG. 10, the dashed rectangle represents a transition zone, in which the magnetic flux changes direction from up to down. Thus, the magnetic flux in the transition zone is not vertical to the surfaces of the permanent magnets. This non-vertical magnetic flux will cause undesired force Ft and undesired torque Mx.
FIG. 3a illustrates a conventional dual magnet VCM 20. The conventional dual magnet VCM includes a top plate 21, a bottom plate 22 arranged in spaced relation and parallel to the top plate 21. Two pieces of permanent magnets 23, 24 are attached to the plates 21 and 22, respectively, such that an air gap is defined between the magnets. A movable coil 251 is positioned in a magnetic field defined between the two pieces of permanent magnets 23, 24. Each piece of the permanent magnets 23, 24 is divided into two halves, which are magnetized in opposite direction.
FIGS. 3b-3d illustrate the magnetic flux generated in the conventional dual magnet VCM by dashed arrows. Referring to FIG. 3b, in the air gap between the two permanent magnets 23, 24, lines of magnetic flux is almost vertical to the surfaces of the permanent magnets 23, 24. When a predetermined driving current flows through the movable coil 251, driving force Fr represented by the solid arrow is generated on the movable coil 251 according to the Fleming's left-hand rule by the interaction between the current and a magnetic field formed by the permanent magnets 23, 24, thereby driving the actuator arm to move. As shown in FIG. 3b, the transition zone of the dual magnet VCM is small and the vertical forces Ft generated on the coil 251 which cause undesired torsion Mx can cancel with each other. Put another way, when a predetermined driving current flows through the movable coil 251, the dual magnet VCM 20 suffers tiny or no torsion, as shown in FIG. 3e. 
However, the cost of the dual magnet VCM is too high, as each dual magnet VCM must consume two pieces of permanent magnets. With the increase of the price of the permanent magnet, the HDD companies are trying to reduce the consumption of magnet material so as to lower the cost of VCM. Under this condition, single magnet VCM that consumes only one piece of permanent magnet has come into being.
Referring to FIG. 4a, the basic structure of a traditional single magnet VCM 30 is similar to the dual magnet VCM 20 except that there is only one piece of permanent magnet 33 attached to the bottom plate 32 as a total in the traditional single magnet VCM 30. The permanent magnet 33 is also divided into two halves that are magnetized in opposite direction. The work principle of the traditional single magnet VCM 30 is also similar to that of the conventional dual magnet VCM 20. When a predetermined driving current flows through the movable coil 351, a driving force Fr represented by the solid arrow as shown in FIG. 4b is generated on the movable coil 351 according to the Fleming's left-hand rule by the interaction between the current and a magnetic field formed by the magnet 33 and the plates 31, 32, thereby driving the actuator arm to move.
FIGS. 4b-4d show lines of magnetic flux generated in the traditional single magnet VCM 30. Although this VCM can lower the cost, the transition zone of the traditional single magnet VCM 30 is much larger than that of the dual magnet VCM 20 shown in FIG. 3b. Moreover, the vertical forces Ft generated on the coil 351 at opposite sides of the X-axis are at different direction, as best shown in FIG. 4e. These opposite vertical forces Ft cause a high torsion on the coil 351.
Referring to FIG. 11, the figures prove that the traditional single magnet VCM suffers a serious torsion Mx.
Hence, a need has arisen for providing an improved VCM capable of reducing the cost of the VCM as well as the torsion generated on the coil of the VCM.